In the drilling and completion of an oil or gas well, a cement composition is introduced to the well bore for cementing pipe string or casing in a process known as “primary cementing”. The cement composition is pumped into the annular space between the walls of the well bore and the casing and sets in the annular space, supporting and positioning the casing, and forming a substantially impermeable barrier, or cement sheath, which isolates the well bore from subterranean zones.
A primary component of this cement composition is Portland cement. Worldwide, it is estimated that 1.8 billion tonnes of Portland cement are produced annually, making it one of the most widely used products in the building and construction industry. The firing process that creates the active cement “clinker” accounts for around one tonne of CO2 emissions to the atmosphere for every tonne of Portland cement manufactured. As a consequence, the use and manufacture of Portland cement is responsible for about 5% of anthropogenic greenhouse gases emitted to the atmosphere. While the production of Portland cement is damaging to the environment, it is currently the raw material of choice in the construction industry for the preparation of concrete and also in the oil and gas well cementing industry. This results in a heavy demand for Portland cement, which has led on occasion to shortages of supply and cost increases. Products, known as “extenders” are available that can be added to a cement mix as a replacement for Portland cements, however, introduction of some of these materials, such as clays, may result in a deterioration of set cement properties, including loss in compressive strength.
There exists a need for a material that is widely available, cost effective and environmentally friendly that can be added to oil and gas well cement. Such a material must not cause a substantial deterioration in the physical properties of the set cement, and ideally it would enhance the physical and mechanical properties of the set cement.
Sulfur is a known component of certain concretes with construction and industrial applications. Sulfur concrete is similar in final appearance to Portland cement concrete, however, its manufacture, handling and testing are different, in part because it does not contain Portland cement or a pozzolanic material.
Sulfur concrete is created by mixing molten sulfur with aggregate at 132-141° C. and allowing the mixture to solidify upon cooling. Ultimate strength is reached in a short time. It exhibits favorable fatigue properties and has excellent resistance to acids, salts, and many organic compounds. It works well as a rapid runway repair material. Sulfur concrete also has unfavorable properties. It has poor durability when exposed to large temperature changes and to wet curing conditions. The material is also brittle.
By general definition, a cement is a binder; a substance which sets and hardens independently, and can bind other materials together. The term “sulfur cement”, used in the prior art dealing with sulfur concrete, refers to sulfur which has been used as a binder for aggregates such as sand, gravel, stone chips or ballast, as well as lightweight aggregates such as pumice and tuff. None of the sulfur cements reported to date contain Portland cement.
A process of modifying sulfur by reacting it with olefinic hydrocarbon polymers can be used for sulfur cements. A similar reaction yields a sulfur soluble polymer concentrate which has been mixed with aggregate to form sulfur concrete, which has been commercially available.
U.S. Pat. No. 4,025,352 issued on May 24, 1977, describes the manufacture of sulfur concrete. The inventors report the manufacturing of NS/ND 22 sulfur concrete, for example, by premixing 95 parts by weight sulfur and 5 parts by weight of liquid dicyclopentadiene at 135° C. and the premix was left at this temperature for 135 minutes. The 100 parts of liquid binder is then poured onto 354.5 parts by weight of DIN 1,164 standard sand, which was at the same temperature (135° C.) and mixing was continued for 15 minutes.
Sulfur concrete does not contain Portland cement and it requires sulfur rather than water as a binder. It cannot be prepared under normal temperatures or by using conventional equipment. It is therefore, apparent that known forms of sulfur concrete are not suitable for oil well cementing applications.
The use of sulfur as an oil and gas well cement presents potential problems associated with the use of sulfur in a downhole environment. It could be expected that elemental sulfur would be inert in the presence of inorganic cementitious materials, and therefore, its addition to a cement blend in any significant quantity could result in a deterioration in mechanical cement properties. Important cement properties include rheology, thickening time, transition time, flexural strength and compressive strength. In order to be useful for oil well cementing, cement blends containing extender type materials must be viable in terms of all of these physical properties.